// k4

t:(+(,`n)!,!10)!+(`p`d!(0 0 0 2 2 2 4 4 7 7;10?100))            / tree (as key table)
t:![t;,(in;`n;`p);0b;(,`d)!,0N]                                 / null out non-leaves
f:50<                                                           / predicate
s:?[t;,(in;`n;?,/(. t[;`p])\'?[t;,(f;`d);();`n]);0b;()]         / select subtree

\

q)s
n| p d
-| ----
0| 0
1| 0 67
2| 0
3| 2 73
4| 2
6| 4 53
7| 4
8| 7 83